U.S. patent application number 09/961779 was filed with the patent office on 2003-03-27 for strip neutral density filter for correcting intensity non-uniformity in a raster output scanning system.
This patent application is currently assigned to Xerox Corporation. Invention is credited to Wang, Mark Shi.
Application Number | 20030058333 09/961779 |
Document ID | / |
Family ID | 25504987 |
Filed Date | 2003-03-27 |
United States Patent
Application |
20030058333 |
Kind Code |
A1 |
Wang, Mark Shi |
March 27, 2003 |
Strip neutral density filter for correcting intensity
non-uniformity in a raster output scanning system
Abstract
A strip neutral density filter varies the Gaussian intensity
profile of the modulated light beam in an overfilled raster output
scanner to provide a generally uniform intensity light beam spot at
the photosensitive medium. The strip neutral density filter is
positioned in the light beam path between the emitting laser source
and the rotating polygon mirror of the raster output scanner.
Inventors: |
Wang, Mark Shi; (Irvine,
CA) |
Correspondence
Address: |
Patent Documentation Center
Xerox Corporation
Xerox Square 20th Floor
100 Clinton Ave. S.
Rochester
NY
14644
US
|
Assignee: |
Xerox Corporation
|
Family ID: |
25504987 |
Appl. No.: |
09/961779 |
Filed: |
September 24, 2001 |
Current U.S.
Class: |
347/261 |
Current CPC
Class: |
G02B 26/124
20130101 |
Class at
Publication: |
347/261 |
International
Class: |
B41J 027/00 |
Claims
What is claimed is:
1. A raster output scanning system in an overfilled polygon design
comprising: a light source for emitting a light beam having a
Gaussian intensity profile; a photosensitive medium; a rotating
polygon mirror having a plurality of reflective facets, said
rotating polygon mirror being located in the path of said light
beam from said light source to said photosensitive medium,
successive reflective facets scanning said light beam across said
photosensitive medium; a neutral density filter being located in
the path of said light beam from said light source to said rotating
polygon mirror, said neutral density filter variably reducing said
Gaussian intensity profile such that said light beam has a
generally uniform beam intensity at said photosensitive medium.
2. The raster output scanning system in an overfilled polygon
design of claim 1 wherein said neutral density filter and said
successive reflective facets form said generally uniform beam
intensity of said light beam at said photosensitive medium.
3. The raster output scanning system in an overfilled polygon
design of claim 1 wherein said neutral density filter has a first
transmissive section, a second transmissive section, and a central
neutral density filter section such that said central neutral
density filter section is positioned between said transmissive
section and said second transmissive section.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to the intensity of the light
beam in a raster output scanner (ROS) and, more particularly, to a
strip neutral density filter for correcting the non-uniformity of
the intensity of the light beam at the scan line in a raster output
scanner having an overfilled facet design.
[0002] Many raster output scanners (ROS) use a rotating polygon
having flat reflective surfaces, or facets, as the scanning element
to form modulated scan lines on the surface of a photosensitive
medium. In a typical system, a beam, modulated according to an
input video data signal, is emitted from a light source such as a
diode laser. The modulated light beam is directed through
pre-polygon conditioning optics, onto the facets of the rotating
polygon mirror. The polygon mirror rotates in the 3 to 30 krpm
range, thus scanning the reflected beam through a post-polygon
optical system and imaging the light beam spot as a scan line
across the full width of a photosensitive medium moving in a scan,
or fast-scan, direction. Meanwhile, the photosensitive medium is
advanced relatively more slowly in a slow scan direction which is
orthogonal to the fast scan direction. In this way, the light beam
scans the medium with a plurality of scan lines in a raster
scanning pattern. The light beam is modulated in accordance with
the video data signals such that individual picture elements
("pixels") of the image represented by the data are exposed on the
photosensitive medium to form a latent image, which can then be
developed and transferred to an appropriate image receiving medium
such as paper.
[0003] In ROS systems there are typically two scanning modes. In a
first mode, pre-polygon conditioning optics incorporate an
underfilled design; e.g. the light from the laser is collimated to
a beam width in the fast scan direction that is smaller than the
polygon mirror facet, typically by a factor of approximately 3. The
underfilled design has been generally preferred in the past because
of a high throughput efficiency and uniform illumination of the
imaging facet.
[0004] A second mode is the overfilled design where the light beam
is collimated to a beam width in the fast scan direction that is
larger than the polygon mirror facet by a factor of approximately 3
in the fast scan direction. In an overfilled design, the facet size
required to produce a given spot size at the photosensitive medium
is greatly reduced allowing more reflective facets to be
accommodated on the same diameter polygon mirror. This, in turn,
permits the ROS system to form more scan lines per second with a
given polygon motor, or, alternatively, to permit the use of less
powerful and less expensive polygon motor drives.
[0005] The overfilled design has several disadvantages. The
throughput efficiency is relatively low (20%), compared to the 50%
efficiency of the underfilled design, and the illumination of the
imaging facet is not as uniform as the underfilled design.
[0006] In an overfilled ROS, the light beam will be incident on and
reflected from more than one facet of the rotating polygon mirror.
The light beam has a Gaussian intensity profile. As the polygon
mirror rotates to scan the light beam spot across a photosensitive
medium, the amount of light reflected to the photosensitive medium
varies because the facets are sampling different parts of the
Gaussian illumination profile and the effective area of the
reflective polygon mirror is changing. The purpose of any ROS is to
provide a uniform intensity profile light beam spot on the
photosensitive medium.
[0007] An aspheric lens system can be used to flatten the Gaussian
profile of the modulated light beam as taught in U.S. Pat. No.
4,941,721, commonly assigned as the present application and herein
incorporated by reference. The aspheric lens system does require
expensive, precision fabricated optical components though.
[0008] A variable transmission filter is used to correct for
polygon mirror error as disclosed in U.S. Pat. No. 5,539,441. The
filter raises the intensity at the edges of the Gaussian profile of
the modulated light beam to correct for facet to facet jitter at
the scan line.
[0009] A liquid crystal window at the output of a ROS adjacent to
the scan line on the photoreceptor can be used to correct the
nonuniformity of a light beam generated by the ROS is disclosed in
U.S. Pat. No. 5,745,155, commonly assigned as the present
application and herein incorporated by reference. The patent also
teaches the use of a long narrow neutral density filter adjacent to
the scan line to attenuate the center of the scan line without
attenuating the Gaussian intensity profiles of the individual spots
that make up the scan line. However, in this patent, the light beam
is a moving scanning beam which has been reflected from the
rotating polygon mirror.
[0010] It is an object of this invention to provide a strip neutral
density filter in the pre-polygon optics of an overfilled ROS to
correct the non-uniformity of the intensity of the light beam.
SUMMARY OF THE INVENTION
[0011] According to the present invention, a strip neutral density
filter varies the Gaussian intensity profile of the modulated light
beam in an overfilled raster outputs scanner to provide a generally
uniform intensity light beam spot at the photosensitive medium. The
strip neutral density filter is positioned in the light beam path
between the emitting laser source and the rotating polygon mirror
of the raster output scanner.
[0012] The strip neutral density filter has a light absorbing edge
and a central neutral density filter section between two light
transmissive side sections. The strip neutral density filter will
block the edges of the incident Gaussian intensity profile light
beam, transmit the side slopes of the incident Gaussian intensity
profile light beam, and reduce the peak Gaussian intensity profile
light beam to a uniform intensity profile.
[0013] Other objects and attainments together with a fuller
understanding of the invention will become apparent and appreciated
by referring to the following description and claims taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a top view of a raster output scanner having an
overfilled facet design with a strip neutral density filter in the
pre-polygon optics for correcting the non-uniformity of the
intensity of the light beam of the present invention.
[0015] FIGS. 2(a) through 2(e) are a perspective view of the strip
neutral density filter of FIG. 1 and the Gaussian intensity profile
light beam from the light source, the filtered intensity profile
light beam after the strip neutral density filter and the generally
uniform intensity profile light beam at the photosensitive
medium.
[0016] FIG. 3 is a graph showing the relative uniformity of light
beam intensity with and without the strip neutral density filter of
FIG. 1 along the photosensitive medium.
DESCRIPTION OF THE INVENTION
[0017] Reference is now made to FIG. 1, wherein there is
illustrated the raster output scanner 100, having an overfilled
polygon facet design with a strip neutral density filter 102 for
correcting the non-uniformity of the Gaussian intensity profile of
the modulated light beam in accordance with this invention.
[0018] In the ROS system 100, video signals serially modulate the
laser source 104 to emit an intensity modulated light beam 106. The
video signals generically represent pixel data for modulating the
light beam 106 on and off to form spots (or pixels) across the scan
line of the photosensitive medium.
[0019] The light beam 106 is collimated by collimating lens 108.
The collimated modulated light beam 106 is filtered by strip
neutral density filter 102.
[0020] As best seen in FIG. 2a, the collimated light beam 106 has a
Gaussian intensity profile 110.
[0021] The strip neutral density filter 102 as shown in FIG. 2b has
a central neutral density filter section 112 adjacent on each side
to a first light transmissive side section 114 and a second light
transmissive side section 116. The first light transmissive side
section 114, the central neutral density filter section 112 the
second light transmissive side section 116 are surrounded by a
light absorbent edge 118 to form the strip neutral density filter
102.
[0022] The collimated light beam 106 of FIG. 2a is incident upon
the strip neutral density filter 102 of FIG. 2b. The edges 120, 122
of the Gaussian profile 110 of the light beam 106 are blocked by
the optical stop of the light absorbent edge 118 of the strip
neutral density filter 102, so that the filtered intensity profile
124 as shown in FIG. 2c has truncated edges 126 and 128.
[0023] The first side slope 130 of the Gaussian profile 110 of the
light beam 106 will be transmitted uneffected by the first light
transmissive side section 114 of the strip neutral density filter
102 so that the filtered intensity profile 124 has a first Gaussian
side slope 132.
[0024] Similarly, the converse of the first side slope 130, the
opposite second side slope 134 of the Gaussian profile 110 of the
light beam 106 will be transmitted uneffected by the second light
transmissive side section 116 of the strip neutral density filter
102 so that the filtered intensity profile 124 has a first Gaussian
side slope 136.
[0025] The central upper slopes 138 and 140 and the peak 142 of the
Gaussian profile 110 of the light beam 106 will be reduced in
intensity by the central neutral density filter section 112 of the
strip neutral density filter 102 to a central flat uniform
intensity plateau profile 144 in the filtered intensity profile
124.
[0026] The uppermost point 146 of the first Gaussian side slope 132
of the filtered intensity profile 124 has the same intensity level
as the uppermost point 148 of the second Gaussian side slope 134 of
the filtered intensity profile 124. Both points 146 and 148 have an
intensity level 75% to 25% of the peak intensity 142 of the
incident Gaussian profile 110 of the light beam 106. In this
illustrative example of FIG. 2, the uppermost points 146 and 148
after filtering have an intensity level 60% of the original peak
intensity 142.
[0027] The central flat uniform intensity profile 142 between the
uppermost point 146 of the first Gaussian side slope 132 and the
uppermost point 148 of the second Gaussian side slope 134 has an
intensity level 90% to 40% of the adjacent uppermost points 146 and
148. In this illustrative example of FIG. 2, the central flat
uniform intensity profile 142 has an intensity level 65% of the
adjacent uppermost points 146 and 148.
[0028] The exact intensity levels are dependent upon ROS parameters
and requirements for the intensity of the light source and light
beam intensity at the photosensitive medium.
[0029] The incident light beam 106 emitted by the light source 104
has a Gaussian intensity profile 110. After filtering by the strip
neutral density filter 102, the resulting intensity profile 124 of
the filtered beam 150 has a truncated edge 126, a first Gaussian
side slope 132, a central flat uniform intensity 142, a second
Gaussian side slope 134 and a truncated edge 128. The central flat
uniform intensity plateau has a lower intensity that the uppermost
points 146, 148 of the Gaussian side slopes 132, 134.
[0030] Returning to FIG. 1, the filtered beam 150 from the strip
neutral density filter 102 is focused by the cylindrical lens 152
onto the facets 154A, 154B and 154C of the polygon mirror 156. The
ROS 100 of the present application has an overfilled design so that
the light beam 150 will illuminate more than one facet.
[0031] The cylindrical lens 152 creates a focused beam 150 in the
cross-scan plane at the polygon mirror 156, while maintaining the
collimation of the beam 150 in the perpendicular or scan plane. The
collimated uniform intensity beam 150 has a width in the scan
direction, which will overfill a single facet and illuminate facets
154A, 154B and 154C in sequence. Facet 154A is shown for
illustrative purposes as the imaging facet that reflects the light
beam towards the photosensitive medium, while facets 154B and 154C
are adjacent facets which will reflect the light beam 150 as the
polygon mirror 156 rotates.
[0032] Returning to FIG. 2, the filtered intensity profile 124 of
FIG. 2c will be reflected and convolve from overfilled facets 154A,
154B and 154C of FIG. 2d in sequence to form a flat intensity
profile 158 of FIG. 2e for the light beam 160 upon reflection from
the polygon mirror 156 and at the photosensitive medium.
[0033] Returning to FIG. 1, as the polygon mirror 156 rotates, a
uniform intensity light beam 160 is reflected from overfilled facet
154A and scanned through a post-polygon optical system which
includes f-theta lens assembly 162. Lens assembly 162 includes
either a toroidal f-theta lens pair 164 and 166 or a non-toroidal
f-theta lens plus a cylinder mirror (not shown).
[0034] The f-theta lens assembly 160 focuses the light beam 158 to
a generally circular spot 168 with a generally uniform intensity
profile 158 on the scan line 170 of the photosensitive medium
172.
[0035] The rotating overfilled facets of the polygon mirror 158
cause the spot 168 to sweep across the photosensitive medium 172
forming a succession of scan lines. The scan line 170 lies in the
fast scan direction. In addition, as the facets 154A, 154B and 154C
are rotated, photosensitive medium 172 moves in a slow scan
direction, substantially perpendicular to the fast scan direction,.
Movement in the slow scan direction is such that successive
rotating overfilled facets of the polygon 158 form successive scan
lines that are offset from each other in the slow scan
direction.
[0036] In this way, the light beam scans the photoesensitive medium
with a plurality of scan lines in a raster scanning pattern such
that individual picture elements ("pixels") of the image
represented by the video data signals form a latent image on the
photosensitive medium, which can then be developed and transferred
to an appropriate image receiving medium such as paper.
[0037] As seen in the graph of FIG. 5, the Gaussian intensity beam
profile 110 of light beam 106 has a power uniformity falloff of
10.5 percent across the photosensitive medium while the filtered
intensity profile 124 of the filtered light beam 150 from the strip
neutral density filter 102 after reflection from the overfilled
facets has a power uniformity falloff of 1.4 percent across the
photosensitive medium.
[0038] The strip neutral density filter 102 creates a more uniform
intensity profile light beam for the overfilled ROS 100 of the
present invention.
[0039] In the overfilled ROS 100, the laser source 104, the
collimating lens 108, the strip neutral density filter 102 and the
cylindrical lens 152 form the pre-polygon optics. The strip neutral
density filter 102 filters the stationary light beam 106 between
the light source 104 and the rotating polygon mirror 158.
[0040] F-theta lenses 162 are designed to provide a linear
relationship between the rotation of polygon mirror 158 and the
deflection of the scanned beam 160 in the scan direction onto the
scan line 170 of the photosensitive medium 172.
[0041] While the invention has been described in conjunction with
specific embodiments, it is evident to those skilled in the art
that many alternatives, modifications and variations will be
apparent in light of the foregoing description. Accordingly, the
invention is intended to embrace all such alternatives,
modifications and variations as fall within the spirit and scope of
the appended claims.
* * * * *